A brief construction of a prior art semiconductor laser device which is appropriate for integration with other electronic elements are shown in FIGS. 2 and 3.
Production of the prior art structure shown in FIG. 2 is started by growing a p type AlGaAs cladding layer 202, a multi-quantum well (MQW) active layer 203, an n type AlGaAs cladding layer 204, and an n type GaAs contact layer 205, successively on a semi-insulating GaAs substrate 201. Thereafter, zinc is selectively diffused to retain an n type region in a stripe shape and to produce p type diffusion regions 208. The n type GaAs contact layer 205 is selectively etched at portions where pn junctions due to the zinc diffusion would be exposed to the surface. Further, electrodes 206 and 207 are produced at the surface portions of the n type and p type layers, respectively.
In the semiconductor laser device produced, the portions of the MQW layer 203 where zinc diffusion takes place are disordered. The disordered regions become AlGaAs layers having uniform composition. This construction results in a so-called buried type laser structure.
This prior art structure of FIG. 2 has pn junctions at the peripheries (left and right side ends) of active region 209 (a region of the MQW layer 203 which is not disordered), Pn junctions produced between the n type AlGaAs layer 204 and the diffusion regions at both sides thereof. Pn junctions are produced between the active region 209 and the p type AlGaAs layer 202. In this case, the former pn junctions have a lower diffusion voltage than the latter pn junctions. When a voltage is applied between the p type and n type electrodes 207 and 206, a current flows through the pn junctions at the peripheries of active region 209 having the lower diffusion voltage, and carriers are injected into the active region 209. Then, since the peripheral portions (left and right side end portions) of the active region 209 are surrounded by AlGaAs layers having a low refractive index as described above, this region becomes a light waveguide. When the width of the waveguide is sufficiently narrow, a stable single mode oscillation is obtained, resulting in a low threshold current. In addition, the fact that both of p type and n type electrodes 207 and 206 are produced on the same main surface makes the device appropriate for integration with other electronic elements.
On the other hand, in the semiconductor laser device of the structure shown in FIG. 3, the MQW layer 209 or 203 is between high resistance AlGaAs layer 301 and 302. P type and n type impurity diffusion regions 208 and 304 are produced to reach both of the MQW layers 203, and these MQW layer regions 203 are disordered similarly as above. In this case, a current flows from the p type diffusion region 208 through the MQW active layer 209 to the n type diffusion region 304. In other words, although the current injection mechanism is a little different from the structure of FIG. 2, a semiconductor laser device appropriate for integration with other electronic elements and having good properties is also obtained.
In the prior art structure of FIG. 2, however, although a current can be effectively injected into the active region, since the refractive index difference between the active region and the AlGaAs layer at the periphery of the active region is relatively large, the width of the active region has to be made quite narrow, such as below about 2 micron, in order to obtain a single mode oscillation. Furthermore, it is quite difficult to produce an n type electrode having a low contact resistance on this narrow portion.
On the other hand, in the semiconductor laser structure of FIG. 3, since current is injected in a direction transverse to the active region, carrier concentration is distributed over some range and this results in low efficiency injection. Furthermore, the resistance value of the element becomes high, and this may restrict the highest temperature at which continuous oscillation may be safely sustained.
In both of the semiconductor laser structures of FIGS. 2 and 3, it is required that the cladding layer be relatively thick, i.e., about 2 micron in order to effectively confine the light in the active layer. It is also required to provide a GaAs layer as a contact layer in order to obtain an electrode of desirable contact properties. This results in a large thickness of about 3 micron from the wafer surface to the active layer, thereby resulting in a long diffusion time for impurity implantation. Furthermore, this results in difficulty in the application of ion implantation techniques and a large depth difference in the integration with other electronic devices.